This invention relates to transmissions, and more particularly to transmission mechanisms which are driven by a variable speed input and provide an output at a nominally constant speed.
Constant speed drives for achieving a constant output speed for driving a load with a varying input speed are well known in the art. An important use of such a constant speed drive is in aircraft for driving a generator or alternator providing a constant frequency electrical power source for the aircraft. The constant speed drive is driven by the aircraft engine which can operate at varying speeds.
Typically, the constant speed drive has a differential with a plurality of relatively movable elements and a trim device associated therewith. This trim device is usually a hydrostatic transmission in the form of a flow-connected hydraulic pump and motor and the hydraulic pump and motor are sized proportional to the speed and load range over which the constant speed drive must operate. There are two basic configurations of the differential transmission mechanism used as a constant speed drive. There can be either an input summed differential or an output summed differential. In each instance, the trim device can add or subtract speed at the differential as required to provide a constant output speed as the input speed varies. In areas of load carrying capability and efficiency, output summed units have distinct advantages at low input speeds while input summed units are better at high input speeds.
In an output summed differential, the working pressure of the trim device is relatively constant across the speed range at any specific generator load. In connection with this, the torque of the trim device is essentially independent of speed, allowing full utilization of the torque capabilities of the trim device, but the speed capability thereof is proportional to the input speed, thereby limiting the speed utilization at the low end of the input speed range.
When an input summed differential is used, the trim device, namely, the hydraulic pump and motor, can operate from maximum speed in one direction of rotation to maximum speed in the other direction of rotation, making maximum use of the speed capabilities of the trim device, but the torque varies inversely with the input speed causing less than maximum utilization of the torque capabilities of the trim device. Additionally, when an input summed differential is operated below straight through speeds, i.e., when the input speed is below the speed where trim speed equals zero, recirculating hydraulic power can potentially cause control problems.
The foregoing characteristics tend to produce the same size trim device for use with either an input or an output summed differential, except that with the input differential, the trim device must also provide the torque to make up its own losses which increase at the low end of the input speed range. In this regard, the leakage losses at the low end of the input speed range of an input summed differential can produce a cascading effect in the recirculating hydraulic power wherein the swash plate/wobbler angle of the variable displacement hydraulic unit is increased to make up for the leakage losses, which results in an increase in the power requirement for the variable displacement hydraulic unit, which in turn results in an increased pressure, which in turn creates additional leakage losses which can then require further increases in the swash plate/wobbler angle, thereby creating a cascading effect. Because of this, the output summed differential is usually used unless some other requirement makes an input summed differential attractive.
While input and output summed constant speed drives have proven quite satisfactory for their intended purposes, there is always room for improvement.
It is a primary object of the invention to provide a new and improved method and apparatus for transmitting drive torque to a dynamoelectric machine at a nominally constant speed from a variable speed engine.
According to one aspect of the invention, a transmission is provided for use between an engine and a dynamoelectric machine to transmit drive torque from the engine to the dynamoelectric machine. The transmission includes an input shaft to transmit drive torque from an engine to a remainder of the transmission; an output shaft to transmit drive torque to a dynamoelectric machine from the transmission; a differential including an input rotary element driven by the input shaft, an output rotary element driving the output shaft, and a third rotary element rotatably coupled to the input and output elements to control the speed ratio between the input and output rotary elements; a first variable displacement hydraulic unit rotatably coupled to the output rotary element to transmit torque between the first hydraulic unit and the output rotary element; a second hydraulic unit rotatably coupled to the third rotary element to transmit torque between the second hydraulic unit and the third rotary element; and a third hydraulic unit driven by the input shaft. The first and second hydraulic units are hydraulically connected in parallel to vary the rotational speed of the second hydraulic unit as a function of the displacement of the first hydraulic unit. The third hydraulic unit is hydraulically connected in parallel to the first and second hydraulic units to transfer hydraulic power thereto.
As one feature of the invention, the third hydraulic unit is a variable displacement hydraulic unit hydraulically connected to the second hydraulic unit to vary a rotational speed of the second hydraulic unit as a function of the displacements of the first and third hydraulic units.
According to one aspect of the invention, a method is provided for transmitting drive torque to a dynamoelectric machine at a nominally constant speed from a variable speed engine. The method includes the steps of transmitting a first drive torque from a variable speed engine to an input rotary member of a differential, transmitting a second drive torque between a first hydraulic unit and an output rotary member of the differential, transmitting hydraulic power between the first hydraulic unit and a second hydraulic unit, transmitting a third drive torque between the second hydraulic unit and a third rotary member of the differential to control the speed ratio between the input and output rotary members, varying the displacement of the first hydraulic unit to vary the rotational speed of the second hydraulic unit and to obtain a nominally constant rotational speed of the output rotary member, transmitting a fourth drive torque to a dynamoelectric machine from the output rotary member, transmitting a fifth drive torque from the engine to a third hydraulic unit, and transmitting hydraulic power from the third hydraulic unit to the first and second hydraulic units.
According to one feature of the invention, the method further includes a step of varying the displacement of the third hydraulic unit as a function of the speed of the engine.
Other objects, features and advantages of the invention will become apparent from the following specification taken in connection with the accompanying drawings.